It would help us to help you better if you told us, exactly how you are getting input from your user. For example, if the user fills in an HTML form, a possible approach is the MAXLENGTH attribute:

<input type="text" name="irah" maxlength="10 value="">
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Otherwise, there are the length and substr functions. Looking at the questions you ask, maybe reading perlfunc and a good introductory book to Perl would be in order?

For example, I am getting user name from stdin. I restricted the user name must be less than 15 characters. So when user try to type 16th character, I won't allow the user to type. How can I achieve in Perl.

You will need to look at Term::Readline or Curses or any of the other "interactive" console things, or you can just implement it yourself using getc.

citromatik reminded me of the module whose name escaped me, Term::ReadKey.

I would handle this by just using the normal chomp($name = <STDIN>), checking its length, and then prompting the user to either accept the truncated name (eg much_too_long_name would become much_too_long_n) or to re-enter a shorter name. A simple looping prompt. You would do something similar if you were looking for a number and the user entered a string.

See conditional statements and length in Perl Documentation. Refer: perlop , perlfuncand perlintro.